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Modeling the Generation of Haida Eddies Modeling the Generation of Haida Eddies E. Di Lorenzo1, M.G. G. Foreman2, W.R. Crawford2 1Scripps Institution of Oceanography, University of California, San Diego, USA 2Institute of Ocean Sciences, Fisheries and Oceans Canada, Sidney, British Columbia, Canada Accepted for publication in Deep Sea Research II Revision January, 2004 1Corresponding author mailing address: Physical Oceanography Research Division Scripps Institution of Oceanography University of California, San Diego 9500 Gilman Drive , La Jolla, CA 92093-0213 Tel: (858) 534-6397 FAX: (858) 534-8561 e-mail: [email protected] FedEx address: 8810 Shellback Way, 439 Nierenberg Hall KEYWORDS: Gulf of Alaska, anticyclonic eddies, Haida eddy, buoyancy flows 1 Modeling the Generation of Haida Eddies E. Di Lorenzo1, M.G. G. Foreman2, W.R. Crawford2 1Scripps Institution of Oceanography, University of California, San Diego, USA 2Institute of Ocean Sciences, Fisheries and Oceans Canada, Sidney, British Columbia, Canada Abstract A numerical model forced with average annual cycles of climatological winds, surface heat flux, and temperature and salinity along the open boundaries is used to demonstrate that Haida Eddies are typically generated each winter off Cape St. James, at the southern tip of the Queen Charlotte Islands of western Canada. Annual cycles of sea surface elevation measured at coastal tide gauges and TOPEX/POSEIDON crossover locations are reproduced with reasonable accuracy. Model sensitivity studies show that Haida Eddies are baroclinic in nature and are generated by the merging of several smaller eddies that have been formed to the west of Cape St. James. The generation mechanism does not require the existence of instability processes and is associated with the mean advection of warmer and fresher water masses around the cape from Hecate Strait and from the southeast. These advected water masses generate plumes of buoyant flow, which intensify and sustain small patches of anticyclonic circulation immediately to the northwest of the cape. When the flow is stronger, several of these smaller eddies can merge to generate a larger eddy, the Haida Eddy. Similar to observations, a typical generation-shedding cycle for larger Haida Eddies in the model is 3-4 months. Consistent with previous in-situ water property measurements, these experiments show that the eddies are generally comprised of mixed layer water from Hecate Strait, Queen Charlotte Sound, and the continental shelves off northern Vancouver Island. Their vertical extent during the mature stage is roughly 1000 m. 1. Introduction A recent study (Crawford et al., 2002) suggests that Haida Eddies are generated off Cape St. James at the southern tip of the Queen Charlotte Islands, British Columbia (Fig. 1) when pressure-driven outflows from Hecate Strait encounter a narrow continental shelf and 2 steep bathymetry. Though these eddies are formed every winter, the precise number generated and their particular size seem to vary with the wind and both near- and far-field alongshore flow conditions. Once they advect into the Gulf of Alaska, the eddies are characterized by anticyclonic rotation, a baroclinic structure extending in depth to at least 1000m, a diameter of order 200km, a sea surface height anomaly of up to 40 cm, and waters that are typically less saline and warmer (below the top 100 m) than the surrounding ocean (Crawford, 2002). The particularly large eddies that were formed during the 1997-98 El Niño were initially identified in sea surface height images produced by the Colorado Center for Astrodynamics Research (http://www-ccar.colorado.edu/~realtime/global-real-time_ssh/). Crawford et al. (2002) present (in Fig. 4) their own analysis of a series of 10-day TOPEX/Poseidon (T/P) and ERS-2 altimetry snapshots showing Haida Eddy development between November 1997 and February 1998. Whitney and Robert (2002) and Mackas and Galbraith (2002) provide overviews of the nutrient and zooplankton characteristics of Haida Eddies. In particular, in situ sampling of the 1998 eddies and their counterparts in more recent years have revealed water properties and larvae that are typical of winter coastal water normally found in Queen Charlotte Sound and Hecate Strait (Batten and Crawford, this issue). As a result, these eddies are thought to play a significant role in the regional ecosystem as they sweep plankton, larvae, and nutrients offshore, thereby reducing the biological productivity on the shelf and increasing the offshore productivity as the eddy decays and releases its contents to the ambient ocean (Crawford, 2003; Johnson et al, this issue; Whitney and Robert, 2002). Two previous numerical studies have addressed the generation of eddies in the Gulf of Alaska. Analysing results from a 1/8º, six-layer, isopycnal model (Hurlburt et al., 1996) simulation of the North Pacific for 1981-94, Melsom et al. (1999) described the generation of large eddies off Baranoff Island and the Queen Charlotte Islands at the end of the 1982- 83 El Nino. Eddies off Baranoff Island are commonly referred to as Sitka Eddies and were first studied by Tabata (1982), while the Queen Charlotte eddies have come to be known as Haida Eddies, in reference to Haida Gwaii, the native name for the Queen Charlotte Islands. Through correlations between pseudo (e.g., from an atmospheric model) wind stress and sea levels measured at the Sitka tide gauge, Melsom et al. (1999) suggested that the 1983 eddies arose predominantly from baroclinic instabilities associated with ENSO teleconnections 3 (Kelvin waves) rather than local wind forcing. However in other years such as 1992, local atmospheric forcing was thought to play a larger role in eddy generation. This study was extended by Murray et al. (2001) using simulations from the same six-layered model but a more refined 1/16º grid. In this case, the main conclusion was that the formation of a coast- wide train of large anticyclonic eddies similar to those described by Thomson and Gower (1998) was caused by baroclinic instabilities associated with the arrival of Kelvin waves at the peak of the annual cycle in the Alaska Current. Although both the Melsom et al. (1999) and Murray et al. (2001) model simulations claimed to have their coastlines defined at the 200 m isobath, Fig. 6 and plate 1 in those respective papers indicate that a more restrictive definition was used along the continental shelf between Vancouver Island and the Queen Charlotte Islands. Despite the fact that Hecate Strait and Queen Charlotte Sound have depths greater than 400 m, both regions seem to be missing from the two model domains. Therefore, in light of the Whitney and Robert (2002) conclusion on the origin of water in Haida Eddies, a re-examination of the eddy generation dynamics described by Melsom et al. (1999) and Murray et al. (2001) would seem to be warranted using a model that includes all the continental shelf. Crawford et al. (2002) (henceforth CCFG02) argue that although eddies generated to the north of Cape St. James may arise from the instabilities described by Melsom et al. (1999) and Murray et al. (2001), it is unlikely that these two models accurately simulate details of the flow near Cape St. James and hence Haida Eddy formation. To substantiate this conclusion, CCFG02 cite the laboratory studies of Cenedese and Whitehead (2000) (henceforth CW00), the existence of a tidal residual eddy west of the cape (Thomson and Wilson, 1987), and current meter observations that show strong southward components in the residual flows both to the east and west of the cape. In particular, persistent southward flows observed to the west of the cape would not be consistent with conditions required for baroclinic instability. Two AVHRR figures in Thomson and Wilson (1987) also show clear anticyclonic eddies west and southwest of Cape St. James. The CCFG02 hypothesis is that strong north-westward winds result in large outflows past the cape, and eddies being formed through mechanisms similar to those described in the CW00 rotating tank experiments. In particular, applying parameter values characteristic of this region, CCFG02 determine that eddies should hug the shore when their radius is about 40km or less, and detach when 4 bigger. This size limit is consistent with both the tidal residual eddies observed and modelled by Thomson and Wilson (1987) and the early Haida 1998 altimeter images. Nof et al. (2002) recently describe an eddy formation mechanism similar to that of CW00. Motivated by the “teddies” generated when Indonesian Throughflow empties into the Indian Ocean northwest of Australia, they show that on a β-plane, eddies detach periodically from an inflow source when their size is sufficiently large that the westward drift exceeds the growth rate. The objective of this study is to carry out numerical experiments to determine the generation mechanism of Haida Eddies. In particular, we wish to confirm or refute the applicability of the mechanisms suggested by CW00, Nof et al. (2002), Melsom et al. (1999) and Murray et al. (2001). The paper is organized as follows. Section 2 provides some details on the numerical model and forcing, Section 3 describes the annual cycle simulations, Section 4 and 5 describes generation and shedding mechanisms through model sensitivity studies, and finally, Section 6 discusses the results and plans for future work. 2. Model and Forcing Details The model used in our studies was the Regional Ocean Modeling System (ROMS), a descendant of SCRUM (Song and Haidvogel, 1994) that is garnering wide usage (Di Lorenzo, 2003; Haidvogel et al., 2000; Marchesiello et al., 2001a; She and Klinck, 2000). The model uses a generalized sigma-coordinate system in the vertical and a curvilinear horizontal grid. The model domain chosen for our application extends from northern Vancouver Island to the Alexander Archipelago of southeast Alaska (Fig. 1) and has an approximate offshore extent of 650 km. There are 20 levels in the vertical with enhanced resolution in the surface and bottom boundary layers and a horizontal resolution of approximately 8 km.
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